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SC339

POWER MANAGEMENT

Applications Information (Cont.)

Now,

We choose C3 = 470nF as the standard value.

Soft-Start Behavior

Atstart-up,V

~0.5V/ms (+/- 25%) for about 800us. The linear ramping is

followedbyaphaseofsmoothsettlingforabout700usatthe

end of which the output has fully settled (to better than 1%).

The total start-up time of about 1.5ms is kept within 1ms

- 2ms window, and this is regardless of the loading and

of the external components connected to the device.

SC339 Start-Up Response

C3 = ½ • • R3 • 3kHz = 530nF

Layout Guidelines

The advantages of using the SC339 to drive external

MOSFETs are:

a) that the bandgap reference and control circuitry are in a

die that does not contain high power dissipating devices

and,

b) that the device itself does not need to be located right

next to the power devices. Thus very accurate output

voltages can be obtained since changes due to heating

effects will be minimal.

The 0.1μF bypass capacitor should be located close to the

supply (IN) and GND pins, and connected directly to the

ground plane.

The feedback resistors should be located at the device,

with the sense line from the output routed from the load

(or top end of the droop resistor if passive droop is being

used) directly to the feedback chain. If passive droop is

being used, the droop resistor should be located next to the

load to avoid adding additional unplanned droop. Sense

and drive lines should be routed away from noisy traces

or components.

For very low input to output voltage differentials, the

input to output/load path should be as wide and short

as possible. Where greater headroom is available, wide

traces may suffice.

Power dissipation within the device is practically

negligible, thus requiring no special consideration dur-

ing layout. The MOSFET pass devices should be laid out

according to the manufacturer’s guidelines for the power

being dissipated within them.